High-frequency directional coupler



Feb. 1, 1955 s. E. MILLER HIGH-FREQUENCY DIRECTIONAL COUPLER 5Sheets-Sheet 1 Filed March 17, 1951 FIG. .3

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ATTORNEY Feb. 1, 1955 Filed March 1'7. 1951 HIGH-FREQUENCY DIRECTIONALCOUPLER 5 Sheets-Sheet 4 k E i U E 5 b g 82k 81 WW lNl/E/VTOR $.EM/LLERATTORNEY Feb. 1, 1955 s. E. MILLER HIGH-FREQUENCY DIRECTIONAL COUPLER 5Sheos-Sheet 5 Filed March 17, 1951 FIG. /5

n 2 E4 7 m m 0 I W n V a 1 3 n E o II ll ll ll ll INVENTOR 5. E. M/LLERBY ATTORNEY United States Patent HIGH-FREQUENCY DIRECTIONAL COUPLERStewart E. Miller, Middletown, N. .L, assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication March 17, 1951, Serial No. 216,132

Claims. (Cl. 33310) This invention relates to electrical wavetransmission systems and, more particularly, to improved electromagneticwave energy couplers providing a directional coupling characteristicbetween two transmission lines, such as wave guides, coaxial lines orthe like.

One of the early practical types of directional couplers was describedin an article in the Proceedings of the Institute of Radio Engineers,February 1947, vol. 35, pages 160 to 165 by W. W. Mumford. The couplersthere disclosed are now well known in the art, and countless uses andapplications thereof have been described in the published art. Ingeneral, all presently known directional couplers are formed by a shortsection of main transmission line coupled to a short section ofauxiliary line. The coupling between the two sections is arranged sothat an electromagnetic wave traveling in a single direction along themain line induces a principal secondary wave, known as the forward wave,traveling in a single direction along the auxiliary line. Likewise, thiscoupling operates so that a wave traveling in the opposite direction inthe main transmission line induces a principal secondary wave travelingonly in the opposite direction in the auxiliary line.

In most practical directional couplers there is also an induced orsecondary wave traveling in the opposite direction from each forwardwave, known as the backward wave. The forward and the backward waves aredesirably greatly unequal in strength. Their relative strength is calledthe directivity of the coupler and is usually expressed as the decibelratio of the forward wave current to the backward wave current. Thestrength of the desired induced forward wave in the auxiliary linecompared to the inducing wave in the main line is called the couplingloss and is also expressed as the decibel ratio of the desired orforward induced wave to the inducing wave in the main line. Thiscoupling loss is actually the transfer ratio between the two lines andthere is no power dissipated in the structure. The performance of adirectional coupler may be described in terms of this directivity andcoupling. To operate satisfactorily, the directivity of a coupler mustexceed some minimum design value at all frequencies within its operatingrange. Thus, the plot of directivity versus the operating frequency orwavelength of the coupled energy is known as the directivitycharacteristic of the coupler and will be so designated herein.

The coupler, as disclosed in one embodiment by Mumford in theabove-mentioned publication, consists of a short section of auxiliarywave guide located contiguous to the main wave guide. Coupling isprovided between the main wave guide and the auxiliary wave guide by apair of longitudinally spaced holes in a common side wall. A travelingwave in the main guide will induce a traveling wave in the auxiliaryguide traveling in the same direction and, since the path length of theenergy coupled through each of the holes is equal, no electricalinterference in the forward direction results. The path lengths of theoppositely directed or backward waves induced through the two holes intothe auxiliary guide are unequal and, due to the spacing of one-quarterwavelength between the holes, cancellation results and no resulting wavewill be induced in the auxiliary guide in the backward direction or in adirection opposite to that of the wave in the main guide.

This results in the directivity of the coupler being frequency sensitivesince the desired high directivity occurs only at the frequency at whichthe coupling holes are separated by one-quarter wavelength. Thisfrequency is known as the design or center frequency. At frequenciesslightly higher and slightly lower than the design frequency thedirectivity decreases, giving a finite operating range over which thedirectivity exceeds the minimum design value.

It is shown in the above-mentioned publication that the frequency rangeover which the directivity exceeds a minimum design value may bebroadened by employing an increased number of coupling elements whichare still spaced one-quarter wavelength apart, but which have theircoupling effects related to each other in accordance with thecoefiicients of a binomial expansion. Other coupler designs have beendisclosed in which the frequency range of directivity has been increasedby providing an infinite series of couplings or a distributed couplingbetween the two guides. This has in general been provided either by aplurality of probes or by a long narrow slot. This coupling is locatedin either case in a side of the guide walls which is perpendicular tothe electric vector of the wave energy, that is, a wider wall assumingdominant mode energy, and provides coupling extending over an unusuallylong distance which must exceed several wavelengths at the operatingfrequency.

It is an object of the present invention to improve the directivitycharacteristic of directional couplers by increasing the directivityover a substantially increased operating frequency range.

It is a further object to shorten the wavelength distance over whichcoupling is required for improved directivity and thereby decrease thephysical size of directional couplers.

In the prior directional couplers, particularly those of the typesmentioned above, in which the coupling is obtained either by one-quarterwave spaced holes, or by distributed coupling in the wider side wall ofrectangular guides, the directivity characteristic and also the couplingloss characteristic is inflexibly tied to the physical couplingmechanism which gives the directional effect. In other words, certaintypes of coupling have been determined to be directive, and to give aninherent directivity characteristic, but it was in no way possible toadjust or change this characteristic if it was not already particularlydesirable.

It is therefore a further object of the present invention to produce indirectional couplers predetermined directivity characteristics havingarbitrarily large or small band width characteristics.

It has been determined, in accordance with the invention, that when theparticular type of coupling, known as divided aperture coupling, to bedescribed with reference to specific embodiments hereinafter, isemployed to relate the two transmission lines, the directivitycharacteristic of the coupler is directly related by a Fourier transformequation, to the shape of the plot of the magni tude of distributedcoupling versus the coupling distance along the region of distributedcoupling. In other words, the directivity is the transform of the shapeof the coupling distribution, and conversely, the coupling distributionshape is the transform of the directivity. While this is truetheoretically and is subject to rigorous mathematical proof, thepractical difiiculty of handling the mathematical relations precludesliteral application of the principle in the design of commercialdirectional couplers. This is so since, with the outstanding exceptionof certain fundamental characteristics designated hereinafter as thebasic geometric shapes, the Fourier transform is an extremelycomplicated function. The transforms of these basic geometric shapes,however, are not so complicated and are relatively familiarmathematically in the art, having been used extensively, for example, inantenna design problems.

By the unusual and novel combination of the coupling distribution of twoor more of these basic geometric distributions, a composite couplingdistribution is obtained in accordance with the invention which is, ineffect, a superposition of the basic distributions. This compositecoupling is easy to handle mathematically, simple to construct inphysical directional couplers, and allows substantially anypredetermined band width and directivity to be achieved withoutexperimentation.

It is a further object of the invention to regulate in a predeterminedmanner the amunot of power coupled in the forward direction oftransmission in directional couplers.

Another object of the invention is to transfer a portion ofelectromagnetic wave energy from one transmission line into another,which portion may vary as desired from a very small fraction of thepower in the one line to complete power transfer thereof.

In accordance with the last two mentioned objects of the invention, ithas been determined that the coupling loss in the forward direction oftransmission in directional couplers in which divided aperture couplingis employed is determined by the size of the coupling distributioncharacteristic. In the specific embodiments to be disclosed, this sizeis ideally varied by either the length over which distributed couplingis maintained by an individual divided aperture, or by the number ofindividual divided apertures of smaller length which is employed.

These and other objects, the nature of the present invention, and itsvarious features and advantages, will appear more fully uponconsideration of the various specific illustrative embodiments, shown inthe accompanying drawings and in the following detailed description ofthese embodiments.

In the drawings:

Fig. 1 illustrates a specific embodiment of a microwave directionalcoupler in which directional coupling is provided by a rectangulardivided aperture;

Fig. 2, given by way of illustration, is a diagrammatic representationof the coupler of Fig. 1;

Fig. 3, given by way of illustration, is a directivity characteristic ofthe type to be expected for the coupler of Fig. 1;

Fig. 4 represents a modification of Fig. 1 whereby directional couplingis provided by a triangular divided aperture in accordance with theinvention;

Fig. 5, given by Way of illustration, is a directivity characteristic ofthe type to be expected for the coupler of Fig. 1 when modified inaccordance with Fig. 4;

Fig. 6 represents a modification of Fig. 1 whereby directional couplingis provided by a composite divided aperture in accordance with theinvention;

Fig. 7 is given by way of explanation of the coupler of Fig. 1 whenmodified in accordance with Fig. 6;

Fig. 8, given by way of illustration, is the directivity characteristicof the type to be expected for the coupler of Fig. 1 when modified inaccordance with Fig. 6;

Fig. 9 represents a modification whereby directional coupling isprovided by two identical divided rectangular apertures in accordancewith the invention;

Fig. 10 represents a modification of Fig. 1 whereby directional couplingis provided by a second species of a composite aperture in accordancewith the invention;

Fig. 11 is given by way of explanation of the coupler of Fig. 1 whenmodified in accordance with Fig. 10;

Fig. 12 shows a generalized composite divided aperture of which theaperture of Fig. 10 is a species;

Fig. 13 is given by way of explanation of the coupler of Fig. 1 whenmodified in accordance with Fig. 12;

Fig. 14 represents a modification of Fig. 1 whereby directional couplingis provided by a third special of a composite aperture in accordancewith the invention;

Fig. 15 represents a modification of Fig. 1 whereby directional couplingis provided by an unequally divided aperture;

Fig. 16, given by way of explanation, is the current couplirsrg functionof the unequally divided aperture of Fig. l

Fig. 17, given by way of explanation, shows the magnitude of forwardwaves in a divided aperture directional coupler as a function of thelength and coupling of the aperture; and

Fig. 18 represents a modification of Fig. 1 whereby the magnitude ofpower transmitted in a forward direction may be controlled.

Fig. 1 shows a directional coupler of the type disclosed and claimed inthe copending application of A. G. Fox, Serial No. 236,556, filed July13, 1951, and similar to a related type disclosed and claimed in thecopending application of A. E. Bowen and W. W. Mumford, Serial No.219,426, filed April 5, 1951. This directional coupler comprises a mainsection 10 of shielded transmission line for guiding wave energy, whichmay be i 't g fil wave guide, as shown, having terminal connections 11and 12 at each of its ends. Located adjacent to the main line 10 andhaving a portion of its length contiguous to a portion of the main line,is an auxiliary shielded transmission line 13 for guiding wave energy,which may be a rectangular wave guide, as shown, and having terminalconnections 26 and 27 at its respective ends. A portion of the adjacentwalls 14 and 24 of each of the two guides 10 and 13, respectively, whichwalls are parallel to the electric vector 15 of wave energy, or thenarrower wall of each guide 10 and 13 assuming normal dominant modeexcitation therein, has been removed providing a slot 16 between theguides when viewed from the direction of the wider wall side. Into slot16 is placed an insert 17 which forms a common wall between the adjacentwave guides 10 and 13. Insert 17 is provided with a divided rectangularaperture 18 providing coupling means between guides 10 and 13. The exactdimensions of the divided aperture 18 will be given in detailhereinafter, but it may now be stated in general, that the longitudinaldimension L is greater than one-half wavelength, while the smallerdimension thereof is, in the usual case, considerably less than one-halfwavelength. The aperture is termed divided since extending parallelacross the transverse or narrower dimension of aperture 18 and dividingit longitudinally into a plurality of smaller spaces 19, is a gridcomprising a plurality of dividers or wire 2%). Wires 20 may be solderedinto recesses placed in insert 17 along the edges of aperture 18 or theresulting structure may be stamped or punched from a blank insert. Thepreferable number of Wires 20, their dimension and the maximum size ofspaces 19 will be discussed in detail hereinafter. Each end of mainguide 10 and auxiliary guide 13 is terminated in its characteristicimpedance as indicated diagrammatically by impedances 21 for guide 13,and 22 and 23 for guide 10. A source 25 of microwave energy is showndiagrammatically connected in series with impedance 23 to terminal 11 ofmain guide 10.

Divided aperture 18 provides a current coupling between the lines 10 and13 which is effectively distributed to a substantial degree along thelength L of the aperture. The magnitude of the current coupling isuniform along this length and is proportional to the transversedimension of the aperture. A single undivided rectangular aperture ofthe same size and shape as the divided aperture 18 does not provide thisdesired coupling. To the contrary the undivided aperture behaves as atransmission line, more or less independently of either of the adjacentwave guides. This is true no matter how thin the common wall thicknessis made since a standing wave is produced in the undivided aperture fromend to end thereof as a result of the high coefiicient of reflection atthe aperture ends. Such an aperture indeed behaves substantially like aresonant single point coupling, and it therefore does not have theparticular attributes, including particularly distributed coupling, tobe described in detail hereinafter, which attributes are necessary tothe practice of the present invention.

In Fig. l the plurality of dividers or wires 20 are placed across thedivided aperture 18 at equal intervals along the length thereof. Thestanding waves which, in the absence of wires 20, would tend to form inaperture 18 are now localized between the wires 20 and destroyed to anextent dependent upon the spacing between them being chosen inaccordance with the following general considerations. Generallyspeaking, if the space 19 between adjacent wires 20, or the longitudinaldimensions of the spaces 19 into which the length of the aperture 18 isdivided, is less than one-half wavelength, no standing waves can besupported. Such a spacing, i. e., merely less than one-half wavelength,would tend to produce discrete couplings located at the center pointbetween the wires 20. However, 'since the amount of departure from thedesired continuous coupling is inversely related to the number of suchdiscrete couplings per wavelength, it is necessary that more than onlythe two couplings per wavelength obtained with this one-half wavelengthspacing, be employed. It has been determined that if the number ofspaces 19 is in the range of three or more per wavelength, a usefulapproximation of continuous coupling is achieved, and if the number ofspaces 19 exceeds eight or more per wavelength, the desired continuouscoupling assumed as a basis for the mathematical analysis givenhereafter is actually realized for all practical purposes.

The transverse dimension of the spaces 19, and therefore of dividingaperture 18, is, of course, limited by the smaller of the transversedimensions of the narrow walls 14 and 24 of the wave guides and 13,respectively, and will therefore usually be less than one-halfwavelength. The exact value of the transverse dimension of the dividedrectangular aperture 18 determines the power coupling from the maintransmission line 10 to the auxiliary transmission line 13. It may beeasily shown that the transfer or power coupling loss through this pathvaries substantially as the square of the transverse dimension of theaperture 18. This fact is a most important attribute of the dividedaperture in the narrow wall, but the full significance of this fact willmore readily be appreciated when certain of the embodiments to bedescribed hereinafter are considered. It should be noted, however, thatsince the power transferred does vary as the square of the transversedimension, the current coupling through the divided aperture variesdirectly as the transverse dimension.

The width of the dividers or wires 20, i. e., the diameter thereof ifthe dividers are wires of circular cross section, does not affect thedirectivity within reasonable limits. In the preferred embodiment of theinvention as shown in Fig. 1, this dimension of the dividers, shown aswires 20, is comparable to and perhaps somewhat less than the commonWall thickness between guides 10 and 13, but the operable minimumdimension thereof is principally controlled by physical considerationssuch as rigidity of the dividers 20. The width of the dividers may beincreased substantially beyond the thickness of the common wall withoutcausing any substantial departure from the desired continuous couplingdiscussed above. The power transferred through the divided aperture,however, is affected by the dimensions of the dividers. For example, ifthe dimension of dividers 20 is increased in the plane of the aperture,i. e., decreasing the area of the spaces 19, or if the dimension ofdividers 20 is increased in the direction of wall thickness which wouldnot change the area of spaces 19, the power transferred is reduced. Itis found that the width of spaces 19 changes the current transfer muchmore rapidly than linearly and as a consequence, for example, a givenaperture when divided into ten spaces provides appreciably more powertransfer than the identical aperture divided into twenty spaces.

As pointed out above, and as shown in Fig. 1, the divided aperture 18 islocated in a common wall of the two rectangular guides 10 and 13 whichwall is in the case of both guides parallel to the electric vector ofwave energy in the guide. current coupling between the guides at eachpoint along the aperture 18 is proportional to the transverse dimensionof the divided aperture 18. It should be noted, however, that thiscurrent coupling relation obtains if the divided aperture 18 is locatedin a common wall parallel to the electric vector in only one guide, thatis, assuming dominant mode excitation, if the common Wall includes onlythe narrower wall of one wave guide. The relation also obtains if thedivided aperture is located in a common wall which is perpendicular tothe electric vectors in both guides, provided that the aperture isdisplaced from the center line of the wider wall in both guides.

The manner in which directional coupling operation is obtained from thestructure of Fig. 1 will most easily be understood upon a considerationof the diagrammatic representation of this structure in Fig. 2. On Fig.2 are shown two identical transmission lines 1 and 2 corresponding,respectively, to lines 13 and 10 of Fig. 1. These transmission lines areassumed parallel and the direction of propagation is along the x-axis.The region in which coupling exists, corresponding to the dividedaperture coupling in the structure of Fig. 1, is confined to theinterval length L and is designated on Fig. 2 by the interval fromAssume further that is described by the function q2(x).

travelthe exciting wave generated by the source 25 is As stated above,in this position the ing to the right in line 2. When all the forwardcurrent elements are summed and referred to the plane of The term Zrepresents the characteristic impedance bf either line and its terminalimpedances 21, 22 and 23. The term Ag represents the guide wavelength ofelectromagnetic energy.

The factor k represents the fraction of the total induced current whichtravels forward in the auxiliary line 1. The factor k is thus a measureof the directionality of the coupling on a differential length basis. Ifall the backuiard current elements are summed and referred to the p anethe equation 2 Item-m] Mae is obtained. The ratio of the forward current(Equation 1) to the backward current (Equation 2) is the directivity ofthe coupler defined above. So long as the phase of the coupling functiont (x) does not change between L L -'g and the forward current elementsall add in phase in line 1. However, the backward current elements addin a form of destructive interference. The backward current expression(Equation 2) is in the form of a Fourier transform. Thus, thedirectivity characteristic is directly related to the coupling function(x) by a Fourier transform of (x). Theoretically, then, it is possibleto de sign a coupling function which would produce any desireddirectivity characteristic.

For specific example, the coupling characteristic of the dividedrectangular aperture 18 of Fig. 1 is a rectangular wave, i. e., themagnitude of the current coupling at each point along the length of theaperture is uniform over the coupling interval L, the length of aperture18, and the magnitude is zero outside this interval. The exact manner inwhich this characteristic is obtained by a divided rectangular aperturesuch as 18 has been explained in detail hereinbefore. In terms of thenotations used above, the function (x) for this coupling characteristicis equal to unity as x varies from Equations 1 and 2 above may thereforebe easily evaluated for this coupling function. In each equation thefacfor k becomes /2 since, as demonstrated by Mumford 1n the abovepublication, one-half of the current coupled by an aperture in the sidewall of a wave guide will travel in each direction. The forward currentexpression of Equation 1 becomes 21rL M and where is the guidewavelength of the electromagnetic energy in both guides. Thus, thedirectivity is given by the ratio of the forward current to the backwardcurrent or the ratio in which Dir.

sin u (6) and a coupling interval of approximately three wavelengths isdesirable in order to obtain broad band directivity of the order of 25decibels.

The nature of the coupling in accordance with the invention as thus fardescribed, and the particular characteristics of this coupling may wellbe summarized at this point, to provide a firm foundation from which toproceed to the description of the more refined embodiments of theinvention hereinafter. Thus, the coupling is provided by what has beentermed, and will continue to be termed hereinafter and in the appendedclaims, a divided aperture. A divided aperture may be considered as anoriginal opening, the perimeter of which defines a given geometricshape, but which original opening has been broken down into many smalleropenings or spaces. If the number of smaller spaces is large, theirexact individual size and shape need not be considered, but ratherattention should be directed to the size and shape defined by theperimeter of the original opening or the divided aperture. This shapewill in general be designated as having a basic geometric shape." Themagnitude of current coupled at any point through the divided aperture,located as described, is directly proportional to the transversedimension of the divided aperture so that the dis tribution along theaperture of the coupled current is identical to the physical shape ofthe divided aperture. This current characteristic will be designated asa basic geometric distribution. The Fourier transform of the basicgeometric distribution, and therefore, the transform of the basicgeometric shape is the characteristic of the backward current in theauxiliary transmission line which characteristic is directly related tothe directivity of the coupler. The total current coupled, whichdetermines the coupling loss in the forward direction, depends upon themagnitude of the transverse dimension of the divided aperture. So longas this magnitude is varied without altering the shape of the dividedaperture, as may be done with the basic geometric shapes consideredherein, the coupling loss may be independently chosen by the transversedimension without affecting the directivity characteristics.

Fig. 4 shows an insert 31 which may replace the insert 17 of Fig. 1 andform the common wall between adjacent wave guides and 13. Insert 31 isprovided with a divided aperture 32 having a basic geometric shape of anisosceles triangle having a base of length L. As with the rectangularaperture 18 of Fig. l, the triangular aperture 32 of Fig. 4 is brokeninto smaller spaces 33 by a plurality of grids or Wires 34. The sameconsiderations with regard to the spacing and number of wires 34 treatedabove in connection with Fig. l obtain in the case of Fig. 4. As pointedout in the paragraph just preceding, the magnitude of the currentcoupling at any point along the length L is directly proportional to thetransverse dimension of the divided aperture at that point. Therefore.the coupling distribution characteristic of aperture 32 of Fig. 4 takesa form identical to the physical shape of the triangular aperture. Interms of the notations already used, the function (x) over the intervalfrom may be expressed as 2 L s +x) 7) and over the interval 0 to may beexpressed as 2 L M x) s As in the case of a rectangular couplingdistribution characteristic, this triangular coupling distributioncharacteristic is one of the basic geometric shapes for which theFourier transform is well known. Thus, Equation 2 evaluated for thiscoupling gives a backward current which is expressed as 2 K) FL (2 Theforward current will be Thus the directivity is given by Dir.

sin 10 This function is plotted on Fig. 5 which shows that perfectdirectivity is found in the regions in which the coupling length L is anintegral number of guide wavelengths. The locus of minimum directivityfalls off as and a coupling interval of the order of one wavelengthproduces broad band directivity in excess of 25 decibels. By going to alength slightly greater than two wavelengths, 35 decibels directivitycan be maintained over a very broad band. A comparison with thecharacteristic of Fig. 3 will, therefore, show that the aperture shapeof Fig. 4 has provided a substantial improvement in the directivitycharacteristic with a much shorter required coupling interval.

It has thus been shown with reference to two basic geometric couplingaperture shapes, i. e., rectangular and triangular, that the backwardcurrent and thus directivity characteristic of of the resulting coupleremploying that aperture in divided form is related by the Fourier transform to the shape of the distributed current coupling along the lengthof the divided aperture which is in turn directly related to thetransverse dimension of the aperture. Other basic geometric shapesincludes a one-half period of a cosine wave, a whole period of a cosinewave measured from one minimum point to another and which ismathematically the same as one-half period of a sin wave, and positiveand negative exponential waves. For each of these shapes the Fouriertransform is well known in the art and is expressible in its simplestform as a function of a single angular variable, for example, thevariable u in Equation 4 for the basic rectangular geometrical shape andthe variable in Equation 9 for the basic triangular geometrical shape.Use of the transform will give the evaluation of Equation 2 for thebackward current when a divided aperture, physically of the shapeconsidered, provides the coupling between the main guide and theauxiliary guide.

Merely for the convenience of those who may practice the invention, thefollowing relations for certain of these basic geometric shapes aregiven herein. By no means is this listing to be considered as exclusiveof other basic shapes for which Fourier transforms are functions of asingle angular variable and can readily be derived by those familiarwith the principles of mathematical analysis.

Thus, for a one-half period cosine wave for which the coupling function(and the aperture shape) is tog over the interval and the backwardcurrent (transform of the wave form of q (x)) is The same may be shownfor a positive exponential wave and for a negative exponential wave. Ineach of the above, the functions It and F are the same as defined above.

In each of the above-described coupling distributions, it will be notedthat the single angular variable in each case depends upon the length ofthe coupling interval as this length appears in the function it.However, n accordance with an important feature of the present1nvention, two or more of these basic geometric shapes are combined toform a composite shape. One such composite shape is shown in Fig. 6,wherein an insert, su1 table for disposition in slot 16 of Fig. 1, isprovided with a divided aperture 42 in the shape of two pyramrdedrectangles. In other words, a lower rectangular aperture of longitudinaldimension L and transverse d1mens1on of unity, is located immediatelyadjacent'to and below an upper rectangular aperture of longitudinaldimension kL and transverse dimension 0, the quantity bemg an arbitraryconstant and the quantity k designating a fraction of the length L. Thetwo rectangularapertures are positioned so that a side kL is locatedcontiguous to and centered upon a side of the length L. Thus, the twoseparate rectangular apertures merge into a single cornposite dividedaperture, that is, divided by grids 43, wh ch will provide a couplingqo(x) equal to unity over the intervals from the coupling function t (x)is equal to 1+0.

It may be shown that the Fourier transform for such a pyramid-shapedarea is the sum of the transforms of each of the related rectangularareas. In other words, the

transform of the composite coupling distribution is the sum of thetransforms of each of the basic geometric distributions and is afunction of both of the separate single variables of the basictransforms. The composite coupling, therefore, represents a distributionthe Fourier transform of which expressed in its simplest form is afunction of a plurality of angular variables. Therefore, the totalbackward current resulting from the composite coupling distribution ofFig. 6 may be obtained by independently substituting the propertransforms in Equation 2 for the two component constant amplitudecouplings and adding the two backward currents thus obtainedarithmetically. The total backward current expression for a directionalcoupler employing the aperture shown in the insert of Fig. 6 thenbecomes FL sln u ck sm (ku) The first term of Equation 17 is a functionof a first angular variable a and represents the component backwardcurrent resulting from constant amplitude coupling of the lower basicdistribution of length L and height unity. The second term of Equation17 is a function of a second angular variable ku and represents thecomponent backward current resulting from the upper baic distribution ofheight 0 and of length kL.

On Fig. 7 the magnitude of these two component backward currents areplotted versus the ratio of the coupling interval L to the wavelength ACurve 41 is the current component expressed by the first term ofEquation 17, and curve 42 is the current component expressed by thesecond term of Equation 17. The sum of curves 41 and 42, therefore,represents the total backward current, expressed by Equation 17, of thecomposite distribution of Fig. 6. Whenever the backward currentcontributions resulting from each of the two basic geometricdistributions, i. e., the currents represented by curves 41 and 42, areequal in magnitude and opposite in phase, the total backward currentbecomes zero and the directivity of the coupler employing the apertureshown in the insert of Fig. 6 becomes very large.

In accordance with an object of the invention, high directivity isobtained in broad frequency regions by obtaining substantially equalbackward current magnitudes from the two component basic geometriccoupling distributions and, at the same time, arranging these currentsto be of opposite phase or sign. This is easily done with the componentdistribution of Fig. 6. It will be noted that the factor c in Equation17, representing the relative height of the basic geometric shape,determines the relative amplitudes of the backward currents. Likewise,the factor k, representing the relative lengths of the two basicgeometric shapes, determines the relative phase of the backwardcurrents. For example, Fig. 7 shows that the points at which curve 41represents zero backward current are located at those ratios of at whichcurve 42 represents zero backward current are those ratlos of which areequal to integral multiples of 0.5 divided by the factor k. Thus, avariation in the factor k results in an extension or a contraction ofcurve 42 along the abscissa of Fig. 7 and a variation in the factor cresults in an extension or a contraction of the amplitude of curve 42with respect to curve 41.

As shown on Fig. 7, point 46 on curve 42 represents a positive currentequal in magnitude to a negative current represented by point 47 ofcurve 41. Likewise, points 48 and 51 of curve 42 represent currentsequal in magnitude and opposite in phase to the currents represented bypoints 49 and 50 of curve 41, respectively. Thus, for the ratio valuesrepresented by points 43, 44 and 45 of Fig. 7, zero total backwardcurrent is obtained. In the entire region between ratio values 43 and44, substantial cancellation of backward current is obtained, andexceedingly high directivity results over this entire broad band region.A like broad band region will be centered around the ratio representedby point 45.

As has been previously shown, the directivity of the coupler is given bythe ratio of the forward current to the total backward current. In Fig.8 is shown the directivity of a coupler employing the insert of Fig. 6,the backward currents of which have been considered in Fig. 7. Moreparticularly, Fig. 8 shows the directivity of the coupler of Fig. 6 whenis equal to unity and when k is equal to 0.454. A comparison of Fig. 8with the backward current characteristic of Fig. 7 will show that thepoints of infinite directivity on Fig. 8, such as points 51, 52 and 53,correspond to those ratios of coupling length to wavelength on Fig. 7 atwhich the two backward current cornponents were equal in magnitude andopposite in phase. For example, point 51 of Fig. 8 corresponds to theratio indicated by point 43 on Fig. 7. Likewise, points 52 and 53 ofFig. 8 correspond, respectively, to ratios 44 and 45 of Fig. 7. Itshould, therefore, be apparent that many and varied directional bandpasscharacteristics may be obtained by adjusting the values of the constantsc and k in accordance with the principles which have just beendescribed.

Fig. 9 shows an insert, suitable for location in slot .16 of Fig. 1,which is provided with two identical divided rectangular apertures 52and 53. Both divided apertures 52 and 53 are identical to the aperture18 described hereinbefore in. connection with Fig. 1. With regard todetails of the considerations involved in the spacing and location ofthe grids or dividers 54, reference may be had to the discussion ofFig. 1. Each of the apertures 52 and 53 has a longitudinal dimension Land a transverse dimension of unity. The transverse center lines of theapertures are separated by a longitudinal distance d. It should be notedthat for all values of the center line spacing d of less than L, theapertures 52 and 53 will overlap and, therefore, theoretically produce adistribution identical to that shown for the aperture of Fig. 6.Therefore, the following analysis of the characteristics of the apertureof Fig. 9 applies with equal validity to the aperture of Fig. 6 and maybe considered an alternative of the analysis of the aperture of Fig. 6given hereinbefore.

The backward current expression for either divided aperture 52 ordivided aperture 53 is expressed by Equations 4 and given hereinbefore.Combining these equations the backward current from either dividedaperture 52 or 53 may be expressed It may be easily shown that the totalbackward current resulting from both divided apertures 52 and 53 may beexpressed as the function of a plurality of angular variables 21rd I=[2I ][cos M (19) or the second term cos dependent upon the variable iszero. Thus, a first set of frequencies of infinite directivity may beindependently chosen by selecting the proper length L as it appears inthe first term, and a second set by choosing the proper separation d asit appears in the second term. By properly locating these points ofinfinite directivity a large number of useful directivitycharacteristics may be obtained of which the characteristic alreadyshown in Fig. 8 is one example.

The divided apertures 52 and 53 may be replaced by indentical dividedapertures of any of the basic geometric shapes described hereinbefore,including the triangular shape described with reference to Fig. 4.Furthermore,

divided apertures 52 and 53 may be replaced by the composite apertures,such as the one disclosed in Fig. 6, if desired, with the resultingadvantage of an increased number of points of infinite directivity.

Fig. 10 shows an insert, suitable for location in slot 16 of Fig. 1,which is provided with a composite divided aperture 61. The shape ofcomposite divided aperture 61 is a combination in accordance with theinvention of a basic rectangular shape, such as aperture 18 of Fig. 1,located adjacent to a basic triangular shape, such as divided aperture32 of Fig. 4. As with the composite aperture 42 of Fig. 6, the two basicshapes and their coupling distributions are merged to form a singledivided aperture. As shown, the longitudinal dimension of the rectangleand the base dimension of the triangle are adjacent and are equal to L.The maximum transverse dimension of aperture 61 is equal to 1-I-cinasmuch as the transverse dimension of the rectangle is unity and thealtitude of the triangle is c. The composite area is divided by grids 62into smaller spaces in accordance with the considerations detailedhereinbefore. As has been demonstrated, the total backward currentproduced by the coupling of such a composite coupling distribution isthe arithmetic sum of the backward currents produced by each of thecomponent basic distributions. These component backward currents areplotted in Fig. 11. Thus, the backward current component contributed bythe triangular distribution, which is given by Equation 9 hereinbefore,is represented by curve 63 of Fig. 11 which has the same coordinates asFig. 7 hereinbefore. The backward current component contributed by therectangular distribution, which is given by Equation 4 hereinbefore, isrepresented by curve 64 of Fig. 11. The initial amplitude of curve 63 isequal to 0 so that by properly choosing the value of c, the magnitude ofthe positive current represented by point 65 on curve 63, may be madeequal to the magnitude of the negative current represented by point 66on curve 64. The same will, therefore, be true for points 67 and 68 oncurves 63 and 64, respectively. Thus, in the region between the ratiovalues indicated by points 69 and 70 and in the region between points 71and 72, substantial cancellation of the total backward current isobtained.

In Fig. 12 the same two basic geometric shapes employed in Fig. 10 havebeen used to form the composite aperture 73 except that the basedimension of the component basic triangular aperture has been increasedby a factor k. As in the previous figures, the total aperture area hasbeen divided by grids 74. It will be readily appreciated that thecomposite aperture of Fig. 10 is one species of the aperture of Fig. 12in which the factor k is equal to 1. Similarly, the factor k may be aquantity less than unity, in which case the base of the triangulardistribution will be shorter than the longitudinal dimension of therectangular distribution.

A general expression for the total backward current of a compositedivided aperture resulting from the combination of the basic geometricshapes of a rectangle and a triangle, as shown in Fig. 12, may beexpressed sin FL sin u ch 2 I bT- F -W This equation will be seen to bea combination of Equatron 4, expressing the component backward currentof the rectangular distribution, and Equation 9, expressmg the componentbackward current of the triangular distribution and taking into accountthe factors c and k.

In Fig. 13 the component backward current expressed by the first term ofEquation 20 is represented by curve 77, and the component backwardcurrent expressed by the second term of Equation 20, with the factor kequal to the specific value of 2, is represented by curve 76. It will beobserved that in the entire interval between the ratio values of 0.5 and1.0, substantial cancellation of the total backward current will result.The same is also true for the interval between the ratio values of 1.5and 2. Thus, specific bands, each having a width of one-half wavelength,have been obtained over which the directivity is very large for adirectional coupler having divided aperture coupling provided by theinsert shown in Fig. 12.

One final composite distribution should serve to illustrate theprinciples of the invention. Thus, Fig. 14 shows an insert, suitable forlocation in slot 16 of Fig. 1, which is provided with a compositedivided aperture 81. The shape of composite divided aperture 81 is acombination in accordance with the invention of a basic rectangularshape, such as aperture 18 of Fig. 1, located adjacent to the two minimapoints of a basic whole period of a cosine wave, as defined by Equations14, 15 and 16 hereinbefore. As with the other composite aperturesherein, the two basic shapes and their coupling distributions are mergedto form a single divided aperture. As shown, the longitudinal dimensionof the rectangle and the whole period dimension of the cosine wave areadjacent and are equal to L. The maximum transverse dimension ofaperture 81 is equal to 1+c inasmuch as the transverse dimension of therectangle is unity and the total amplitude of the cosine wave is c. Thecomposite arrangement is divided by grids 82 into smaller spaces inaccordance with the considerations detailed hereinbefore. It will benoted that this distribution is similar to the distribution of Fig. 10,except that the linear taper of the triangular distribution of theinsert of Fig. has been replaced by a cosinusoidal taper of Fig. 14.

The total backward current of the composite divided aperture resultingfrom the combination of the basic geometric shapes of a cosine wave anda rectangle, as shown in Fig. 14, may be expressed csinu This equationwill be seen to be a combination of Equation 4, expressing the componentbackward current of the rectangular distribution, and Equation 15,expressing the component backward current of the cosine distribution,taking into account the factor 0. If the factor c is selected byempirical and graphical methods of the type illustrated hereinbefore byFigs. 7 and 8 for the embodiment of Fig. 6, to be equal to the specificvalue of 22.4, it may be shown that a directivity characteristic of morethan 50 decibels may be obtained at all frequencies in which thecoupling length L is at least 1.5 times the wavelength of the coupledenergy.

It should be apparent that further combinations of basic geometricshapes for which the Fourier transform is known, may readily be made bythose skilled in the art in accordance with the principles of theinvention which have been disclosed. The particular directivitycharacteristic desired will be obtained in each case by proportioningthe relative dimensions of the component geometric shapes in the mannerdemonstrated with reference to several specific embodiments herein.

The particular coupling distributions contemplated by the invention havebeen obtained in each of the preceding embodiments by varying the shapeof the coupling aperture in a predetermined manner. In Fig. an insert isshown, suitable for location in slot 16 of Fig. 1, which provides apredetermined coupling distribution by varying the spacing of the gridsor dividers 82 placed across aperture 81 of constant transversedimension. For example, assume that the triangular coupling function ofFig. 16 is desired along the aperture length L. Thus, the spacingof'dividers 82 will be chosen to render the areas 83, 85 and 87, etc.,proportional to substantially the square root amplitudes 84, 86 and 88of the coupling function at points such as 89, 90 and 91, etc. along theaperture length which correspond to the centers of the spaces 83, 85 and87, etc. A composite distribution may be obtained in this manner byvarying the spacing of the divider elements in accordance with a firstpredetermined function and also varying the transverse dimension of theaperture in accordance with a second predetermined function.

The preceding discussion of the invention in connection with the figuresalready considered in detail, has been directed essentially to thedirectional elfects obtained by the disclosed coupling means, i. e.,directed to the character of the backward current in the auxiliarytransmission line. It has been demonstrated that the directivity of aparticular coupling depends upon the shape of the coupling distributioncharacteristic. It has been noted in passing, however, that conductionin the forward direction in the auxiliary transmission line depends uponfactors which do not affect this shape of the coupling distributioncharacteristic. Further useful advantages may be obtained by increasingthe magnitude of the coupling, either by increasing the length of thecoupling interval or by increasing the number of individual couplingdistributions, and thereby affecting the forward current in the mannerto be described. This is done without varying the shape of thedistribution and, therefore, without changing the directivitycharacteristic. Consider again, therefore, Fig. 1 and particularly thecurrent transmitted in the forward direction in the auxiliary line 13and within a length interval so small that negligible power istransmitted between the lines 10 and 13. The envelope of the travelingwave in the main transmission line 10 may be expressed dz OCE1+ZZE2 andthe traveling wave in the auxiliary transmission line 13 may beexpressed as dz ClEz'i'aE 1 wherein cc represents the continuouscoupling in radians per unit length between the lines. Assuming an inputwave of magnitude unity impressed on the main transmission line and nowave impressed on the auxiliary transmission line, Equation 22 becomesand Equation 23 becomes wherein x represents the distance over which thecoupling is maintained.

The coupling factor a in Equations 22 through 25 is a complexmathematical quantity. In a practical structure, however, therequirement of conservation of energy leads to the restriction thatIEiP-l-[E2l equal a constant for all values of x. Therefore, thequantity or must be a pure imaginary. This results in a degree phasedifference between energy in lines 10 and 11 at all times.

In Fig. 17 the magnitudes of waves on the two lines expressed byEquations 24 and 25 are plotted versus the product of coupling a inradians per unit length times the distance x over which the coupling ismaintained. The wave magnitude in the main transmission line 10represented by curve 92 of Fig. 17 is seen to decline cosinusoidally andthe wave magnitude in the auxiliary transmission line 13 represented bycurve 93 of Fig. 17 is seen to increase sinusoidally as the couplinglength x is increased. Complete power transfer between the lines takesplace when the product [20ml is equal to m1r radians, where m is any oddinteger, and repeats cyclically as long as coupling is maintained. Thecoupling may be broken at any point where the Waves in the two lineshave a relation which it is desired to preserve. In other words, thepower P1 in the main transmission line 10 and the power P2 in theauxiliary transmission line 13 will be divided in accordance with theratio P sin lax] The quantity ax is chosen to provide hybrids with anydesired transfer ratio.

An alternative method for increasing the coupling and controlling theamount of power transmitted in the forward direction is demonstrated inFig. 18 by increasing the number of individual coupling units. Fig. 18shows an insert, suitable for location in slot 16 of Fig. 1, which isprovided with a composite divided aperture 95. The shape of aperture 95is a combination of a plurality of cascaded apertures of the type shownand described hereinbefore with reference to Fig. 6. Only two suchcascaded apertures are shown in Fig. 18, which two will be designatedthe left half of aperture 95 and the right half of aperture 95,respectively, but the number of cascaded apertures may be increased to nas will be shown. Thus, the left half of aperture 95 and the right halfof aperture 95 may be considered as separate discrete couplings eachhaving a good directivity and a determinable coupling loss. Energytransferred from the main transmission line 10 to the auxiliarytransmission line 13 through the left-hand coupling experiences a 90degree phase delay. This energy travels along the auxiliary transmissionline 13 to the righthand coupling and part of this energy returns to themain transmission line 10 with the further phase delay of 90 degrees.Thus, energy which goes from the main transmission line 10 to theauxiliary transmission line 13 and back to the main transmission line 10at a later coupling point arrives in the main transmission line 10 outof phase with the energy which travels straight through in the maintransmission line 10. A summation of such components eventually resultsin cancellation of the wave in the main transmission line 10.

Designating the magnitude of the coupling through either the left halfor the right half of aperture 95 as C, the voltage V1 in the auxiliarytransmission line 13 after the first coupling unit, i. e., the left halfof aperture 95, may be expressed as and the voltage E1 in the maintransmission line 10 as Upon passing the second coupling unit, i. e.,the right half of aperture 95, these voltages become Thus, the requirednumber n of identical coupling units having a coupling factor C may bedetermined for any required ratio in the voltages En and Va. The desiredratio of power division would, therefore, be the square of the voltageexpression. On the other hand, the desired relation between voltage orpower may be obtained by a fixed number of coupling units having therequired coupling factor C. In either case complete power transfer willtake place between the lines when the quantity n sin* C is equal towhere m is any odd integer.

It is desirable in certain applications to cascade a plurality ofdirective apertures having different coupling losses. The desired powerdivision may then be determined as E=cos (ni01+n2l92+n303+ nktik (36)V=sin (n101+n202+ nktik (37) where E and V are the voltages in the mainand auxiliary transmission lines, respectively, at the end of the seriesof couplings, and there are employed:

In apertures of individual coupling Cl 112 apertures of individualcoupling C2 n3 apertures of individual coupling C3 Ilk apertures ofindividual coupling Ck and the transformation has been employed inwriting the above expression for E and V. Equations 36 and 37 may berewritten in the form sin C3+ mg sin- Ck (38) V=sin (m sin C1+n2 sinC2+n3 sin- C3+ mtsin Ck (39) Again complete power transfer between thelines will take place when the expression in the parentheses of Equation39 is equal to in which m may be any odd integer.

It will be apparent that the coupling factors at and C as employed inthe equations for power transfer in the forward direction oftransmission hereinbefore are general expressions of distributed anddiscrete coupling, respectively. For this reason, the relations givenobtain in the case of each type of electrical wave transmission systemknown in the art. The coupling therebetween may be obtained by any ofthe particular coupling means employed in each of these transmissionsystems.

In all cases, it is understood that the above-described arrangements aresimply illustrative of a small number of the many possible specificembodiments which can represent applications of the principles of theinvention. Numerous and varied other arrangements can readily be devisedin accordance with said principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. Directional coupling apparatus for high frequency electrical energycomprising a main length of electrical transmission line, an auxiliarylength of electrical transmission line coupled substantiallydistributively to said main line over a section of the length of saidlines, the length of said section being greater than one-half wavelengthof said energy, a first portion of said section being symmetricallydisposed about the center of said section and having a length less thansaid section, the magnitude distribution of said coupling over saidfirst portion being the sum of the component distribution of one of twodiscontinuous substantially linear tapers and a constant disposedsymmetrically about said center, the magnitude of said coupling over theremaining portions of said section being a continuation of one of thecomponent distributions over said first portion with the other of saidcomponent distributions equal to zero, said first portion and saidremaining portions proportioned so that the maximum amplitudes of thesum of the Fourier transforms of each are less than a value determinedfrom the minimum amplitude of a given directivity characteristic forsaid apparatus.

2. Directional coupling apparatus for electromagnetic wave energycomprising a main shielded transmission line for guiding said waveenergy, an auxiliary shielded transmission line having a portion of itslength contiguous to a portion of said main transmission line, and meansfor coupling said transmission lines with a variation in couplingstrength along said contiguous portion, said means comprising a commonshield in said contiguous portion having a composite aperture therein oflength greater than one-half wavelength of said energy, the shape ofsaid composite aperture being the geometrical combination of at leasttwo basic geometrical shapes that are symmetrical about a transverseaxis of said lines that passes in the plane of said aperture through itscenter, said aperture being asymmetrical about every longitudinal axisof said aperture that passes in the plane of said aperture parallel tothe axis of said lines so that the separate current components coupledfrom said main line into said auxiliary line by each of the compositeshapes of said aperture and which each vary in amplitude and phase asfunctions of the dimensions of one of said shapes are in phase andamplitude to cancel with each other over a substantial band of frequencyvariations of said wave energy.

3. Directional coupling apparatus for electromagnetic wave energycomprising a main shielded transmission line for guiding said waveenergy, an auxiliary shielded transmission line having a portion of itslength contiguous to a portion of said main transmission line, and meanscoupling said transmission lines with a variation in coupling strengthalong said contiguous portion, said means comprising a common shield insaid contiguous portion having an aperture therein, the shape of saidaperture being the geometrical combination of at least two rectangleswhich are each symmetrical about a transverse axis of said lines thatpasses in the plane of said aperture through its center, said rectangleshaving different longitudinal dimensions each greater than one-halfwavelength so that the separate current components coupled from saidmain line into said auxiliary line by each rectangle and whichcomponents each vary in amplitude and phase as functions of thedimensions of one of said rectangles are in phase and amplitude tocancel with each other over a substantial band of frequency variationsof said wave energy.

4. Directional coupling apparatus for electromagnetic wave energycomprising a main shielded transmission line for guiding said waveenergy, an auxiliary shielded transmission line having a portion of itslength contiguous to a portion of said main transmission line, and meanscoupling said transmission lines with a variation in coupling strengthalong said contiguous portion, said means comprising a common shield insaid contiguous portion having an aperture therein, the shape of saidaperture being the geometrical combination of a rectangle and a trianglelocated coextensive with each other along at least a portion of thelength of said contiguous portion so that the separate currentcomponents coupled from said main line into said auxiliary line by saidrectangular portion and by said triangular portion which components eachvary in amplitude and phase as functions of the dimensions of oneportion are in phase and amplitude to cancel with each other over asubstantial band of frequency variations of said wave energy.

5. Directional coupling apparatus for electromagnetic wave energycomprising a main shielded transmission line for guiding said Waveenergy, an auxiliary shielded transmission line having a portion of itslength contiguous to a portion of said main transmission line, and meanscou pling said transmission lines with a variation in coupling strengthalong at least one-half wavelength of said contiguous portion, saidmeans comprising a common shield in said contiguous portion having anaperture therein, the shape of said aperture being the geometricalcombination of a rectangle and a whole period of a cosine wave which areeach symmetrical about a transverse axis of said lines that passes inthe plane of said aperture through its center so that the separatecurrent components coupled from said main line into said auxiliary lineby said rectangular portion and by said cosine portion which componentseach vary in amplitude and phase as functions of the dimensions of oneportion are in phase and amplitude to cancel with each other over asubstantial band of frequency variations of said wave energy.

6. Directional coupling apparatus for high frequency electrical energycomprising a main electrical transmission line, an auxiliary electricaltransmission line coupled distributively to said main line over a givenlongitudinal section of said lines, the length of said section being atleast greater than one-half wavelength of said energy, the plot of thecharacteristic of the magnitude versus distance along said section ofsaid distributed coupling and the distance coordinate of said plotforming an enclosed area, said area requiring more than two parametersto define its two characteristics of size and shape and being divisibleby lines parallel to said distance coordinate into at least two partseach having their two characteristics of size and shape definable byless than three parameters, at least one of said two characteristics ofeach part being diiferent from the corresponding characteristic of everyother part with any of the parts having the same shape having differentlongitudinal lengths so that the separate current components coupledfrom said main line into said auxiliary line by each of said parts andwhich components each vary in amplitude and phase as functions of thesize and shape of said parts are in phase and amplitude to cancel witheach other over a substantial band of frequency variations of said waveenergy.

7. Directional coupling apparatus for electromagnetic wave energycomprising a main shielded transmission line for guiding said waveenergy, an auxiliary shielded transmission line having a portion of itslength contiguous to a portion of said main transmission line, meanscoupling said transmission lines comprising a common shield in saidcontiguous portion having an elongated aperture therein, thelongitudinal dimension of said aperture being greater than one-halfwavelength of said wave energy, the shape of said aperture being thegeometrical combination of a rectangle and a triangle locatedcoextensive with each other along at least a portion of saidlongitudinal dimension so that the separate current components coupledfrom said main line into said auxiliary line by said rectangular portionand by said triangular portion which components each vary in amplitudeand phase as functions of the dimensions of one portion are in phase andamplitude to cancel with each other over a substantial band of frequencyvariations of said wave energy, and a grid comprising a plurality ofwires extending across said aperture and dividing said aperture into aplurality of spaces each having dimensions parallel to said longitudinaldimension of less than one-half wavelength of said wave energy.

8. Directional coupling apparatus for high frequency electrical energycomprising a main electrical transmission line, and an auxiliaryelectrical transmission line, means for coupling said linesdistributively over a given longitudinal section of said lines with thecoupling strength of said means varying with distance along the lengthof said section such that the amplitude of the energy coupled by saidmeans increases by degrees from a first value of zero to a finite valueand subsequently increases to at least one difierent finite value withat least one discontinuity in the degree of said increase between zeroand said different finite value and subsequently returns to zero instages that render the coupling strength characteristic symmetricalabout the center of said longitudinal section, the location of saiddiscontinuity along the length of said section which location determinesthe relative phases of current components coupled from said main lineinto said auxiliary line being selected relative to said finite valueswhich values determine the relative absolute maximum amplitudes of saidcomponents to produce components in said auxiliary line that are inphase and amplitude to cancel with each other over a substantial band offrequency variations of said wave energy.

9. Directional coupling apparatus in accordance with claim 8 in which atleast one of said increases is an abrupt change between two of saidvalues.

10. Directional coupling apparatus in accordance with claim 8 in whichat least one of said increases is an abrupt change between two of saidvalues and at least another of said increases is a tapered increasebetween two of said values.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Montgomery: Technique of Microwave Measurements, RadiationLaboratory Series, vol. II, copyright 1947, pp. 885-890. (Copy in Div.69.)

Publication IA New Type of Waveguide Directional Coupling by Riblet andSaad, published in Proceedings of I. R. E. January 1948.

Publication IISome Possibilities of Heating by Centimetric Power byKeitley, published in the Journal of the British Institution of RadioEngineers in March 1949, pp. 97-121. Only pages 106 and 107 are reliedon.

Publication III-Directive Couplers in Wave Guides by Surdin, publishedin the Journal of the British Institution of Radio Engineers; vol. 93,part IIIA, pp. 726- 736; only page 735 relied on. Published January1947.

